Relatively close at 36.7 light-years from the Sun, Arcturus is a red giant of spectral type K0III—an ageing star around 7.1 billion years old that has used up its core hydrogen and moved off the main sequence. It is 7000108000000000000♠1.08±0.06 times as massive as the Sun, but has expanded to 7001254000000000000♠25.4±0.2 times its diameter and is around 170 times as luminous.

The traditional name Arcturus derives from Ancient Greek Ἀρκτοῦρος (Arktouros) and means "Guardian of the Bear",[9] ultimately from ἄρκτος (arktos), "bear"[10] and οὖρος (ouros), "watcher, guardian".[11] It has been known by this name since at least the time of Hesiod.[12]

The Greek name is a reference to its being the brightest star in the constellation next to Ursa Major, the Greater Bear.

One astronomical tradition associates it with the mythology around Arcas, who was about to shoot and kill his own mother Callisto who had been transformed into a bear. Zeus averted their imminent tragic fate by transforming the boy into the constellation Boötes, called Arctophylax "bear guardian" by the Greeks, and his mother into Ursa Major (Greek: Arctos "the bear"). The account is given in Hyginus's Astronomy.[13]

Aratus in his Phaenomena said that the star Arcturus lay below the belt of Arctophylax, although according to Ptolemy in the Almagest it lay between his thighs.[14]

An alternative lore associates the name with the legend around Icarius, who gave the gift of wine to other men, but was murdered by them, because they had had no experience with intoxication and mistook the wine for poison. It is stated this Icarius, became Arcturus, while his dog, Maira, became Canicula (Procyon), although "Arcturus" here may be used in the sense of the constellation rather than the star.[15]

In 2016, the International Astronomical Union organized a Working Group on Star Names (WGSN)[16] to catalog and standardize proper names for stars. The WGSN's first bulletin of July 2016[17] included a table of the first two batches of names approved by the WGSN; which included Arcturus for this star. It is now so entered in the IAU Catalog of Star Names.[18]

Arcturus is visible from both Earth's hemispheres as it is located 19° north of the celestial equator. The star culminates at midnight on 27 April, and at 9 p.m. on June 10 being visible during the late northern spring or the southern autumn.[21] From the Northern Hemisphere, an easy way to find Arcturus is to follow the arc of the handle of the Big Dipper (Plough). By continuing in this path, one can find Spica, "Arc to Arcturus, then spike (or speed on) to Spica".[22][23]

Ptolemy described Arcturus as subrufa ("slightly red"): it has a B-V color index of +1.23, roughly midway between Pollux (B-V +1.00) and Aldebaran (B-V +1.54).[20]

Eta Boötis, or Muphrid, is only 3.3 light-years distant from Arcturus, and would have a visual magnitude −2.5, whereas an observer on the former system would find Arcturus as bright as Venus as seen from Earth.[20]

Arcturus is moving rapidly (122 km/s) relative to the Solar System, and is now almost at its closest point to the Sun. Closest approach will happen in about 4,000 years, when the star will be a few hundredths of a light-year closer to Earth than it is today. (In antiquity, Arcturus was closer to the centre of the constellation.[12]) Arcturus is thought to be an old disk star, and appears to be moving with a group of 52 other such stars, known as the Arcturus stream.[24]

With an absolute magnitude of −0.30 Arcturus is, together with Vega and Sirius, one of the most luminous stars in the Sun's neighborhood. It is about 110 times brighter than the Sun in visible light wavelengths, but this underestimates its strength as much of the light it gives off is in the infrared; total (bolometric) power output is about 180 times that of the Sun. The lower output in visible light is due to a lower efficacy as the star has a lower surface temperature than the Sun. With a near-infrared J bandmagnitude of −2.2, only Betelgeuse (−2.9) and R Doradus (−2.6) are brighter.

As a single star, the mass of Arcturus cannot be measured directly, but models suggest it is slightly larger than that of the Sun. Evolutionary matching to the observed physical parameters gives a mass of 7030214763400000000♠1.08±0.06 M☉,[7] while the oxygen isotope ratio for a first dredge-up star gives a mass of 1.2 M☉.[29] The star displays magnetic activity that is heating the coronal structures, and it undergoes a solar-type magnetic cycle with a duration that is probably less than 14 years. A weak magnetic field has been detected in the photosphere with a strength of around half a gauss. The magnetic activity appears to lie along four latitudes and is rotationally-modulated.[30]

Arcturus is estimated to be around 6 billion to 8.5 billion years old,[7] but there is some uncertainty about its evolutionary status.[31] Based upon the color characteristics of Arcturus, it is currently ascending the red-giant branch and will continue to do so until it accumulates a large enough degenerate helium core to ignite the helium flash.[7] It has likely exhausted the hydrogen from its core and is now in its active hydrogen shell burning phase. However, Charbonnel et al. (1998) placed it slightly above the horizontal branch, and suggested it has already completed the helium flash stage.[31]

As one of the brightest stars in the sky, Arcturus has been the subject of a number of studies in the emerging field of asteroseismology. Belmonte and colleagues carried out a radial velocity (Doppler shift of spectral lines) study of the star in April and May 1988, which showed variability with a frequency of the order of a few microhertz (μHz), the highest peak corresponding to 4.3 μHz (2.7 days) with an amplitude of 60 ms−1, with a frequency separation of c. 5 μHz. They suggested that the most plausible explanation for the variability of Arcturus is stellar oscillations.[32]

Asteroseismological measurements allow direct calculation of the mass and radius, giving values of 7030159084000000000♠0.8±0.2 M☉ and 7010194100300000000♠27.9±3.4 R☉. This form of modelling is still relatively inaccurate, but a useful check on other models.[33]

Hipparcos also suggested that Arcturus is a binary star, with the companion about twenty times dimmer than the primary and orbiting close enough to be at the very limits of humans' current ability to make it out. Recent results remain inconclusive, but do support the marginal Hipparcos detection of a binary companion.[34]

In 1993, radial velocity measurements of Aldebaran, Arcturus and Pollux showed that Arcturus exhibited a long-period radial velocity oscillation, which could be interpreted as a substellar companion. This substellar object would be nearly 12 times the mass of Jupiter and be located roughly at the same orbital distance from Arcturus as the Earth is from the Sun, at 1.1 astronomical units. However, all three stars surveyed showed similar oscillations yielding similar companion masses, and the authors concluded that the variation was likely to be intrinsic to the star rather than due to the gravitational effect of a companion. So far no substellar companion has been confirmed.[35]

In Arabic, Arcturus is one of two stars called al-simāk "the uplifted ones" (the other is Spica). Arcturus is specified as السماك الرامح as-simāk ar-rāmiħ "the uplifted one of the lancer". The term Al Simak Al Ramih has appeared in Al Achsasi Al Mouakket catalogue (translated into Latin as Al Simak Lanceator).[36]

This has been variously romanized in the past, leading to obsolete variants such as Aramec and Azimech. For example, the name Alramih is used in Geoffrey Chaucer's A Treatise on the Astrolabe (1391). Another Arabic name is Haris-el-sema, from حارس السماءħāris al-samā’ "the keeper of heaven".[37][38][39] or حارس الشمالħāris al-shamāl’ "the keeper of north".[40]

Arcturus was once again called by its classical name from the Renaissance onwards.[41]

The WotjobalukKoori people of southeastern Australia knew Arcturus as Marpean-kurrk, mother of Djuit (Antares) and another star in Bootes, Weet-kurrk[42] (Muphrid).[43] Its appearance in the north signified the arrival of the larvae of the wood ant (a food item) in spring. The beginning of summer was marked by the star's setting with the Sun in the west and the disappearance of the larvae.[42] The people of Milingimbi Island in Arnhem Land saw Arcturus and Muphrid as man and woman, and took the appearance of Arcturus at sunrise as a sign to go and harvest rakia or spikerush.[44]:24,69,112 The Wailwun of northern New South Wales knew Arcturus as Guembila "red".[44]:84

The Mi'kmaq of eastern Canada saw Arcturus as Kookoogwéss, the owl.[46]

Arcturus had several names that described its significance to indigenous Polynesians. In the Society Islands, Arcturus, called Ana-tahua-taata-metua-te-tupu-mavae ("a pillar to stand by"), was one of the ten "pillars of the sky", bright stars that represented the ten heavens of the Tahitian afterlife.[47] In Hawaii, the pattern of Boötes was called Hoku-iwa, meaning "stars of the frigatebird". This constellation marked the path for Hawaiiloa on his return to Hawaii from the South Pacific Ocean.[48] The Hawaiians called Arcturus Hoku-leʻa.[49] It was equated to the Tuamotuan constellation Te Kiva, meaning "frigatebird", which could either represent the figure of Boötes or just Arcturus.[50] However, Arcturus may instead be the Tuamotuan star called Turu.[51] The Hawaiian name for Arcturus as a single star was likely Hoku-leʻa, which means "star of gladness", or "clear star".[52] In the Marquesas Islands, Arcturus was probably called Tau-tou and was the star that ruled the month approximating January. The Māori and Moriori called it Tautoru, a variant of the Marquesan name and a name shared with Orion's Belt.[53]

Prehistoric Polynesian navigators knew Arcturus as Hōkūleʻa, the "Star of Joy". Arcturus is the zenith star of the Hawaiian Islands. Using Hōkūleʻa and other stars, the Polynesians launched their double-hulled canoes from Tahiti and the Marquesas Islands. Traveling east and north they eventually crossed the equator and reached the latitude at which Arcturus would appear directly overhead in the summer night sky. Knowing they had arrived at the exact latitude of the island chain, they sailed due west on the trade winds to landfall. If Hōkūleʻa could be kept directly overhead, they landed on the southeastern shores of the Big Island of Hawaiʻi. For a return trip to Tahiti the navigators could use Sirius, the zenith star of that island. Since 1976, the Polynesian Voyaging Society's Hōkūleʻa has crossed the Pacific Ocean many times under navigators who have incorporated this wayfinding technique in their non-instrument navigation.

In Ancient Rome, the star's celestial activity was supposed to portend tempestuous weather, and a personification of the star acts as narrator of the prologue to Plautus' comedy Rudens (circa 211 BC).[55][56]

Arcturus achieved fame when its light was rumored to be the mechanism used to open the 1933 Chicago World's Fair. The star was chosen as it was thought that light from Arcturus had started its journey at about the time of the previous Chicago World's Fair in 1893 (at 36.7 light-years away, and the light actually started in 1896.[60]

The star is mentioned in the 1977 documentary film Powers of Ten, in which it is seen when a camera zooms from Earth to the whole of the known universe.

^Massarotti, Alessandro; Latham, David W.; Stefanik, Robert P.; Fogel, Jeffrey (2008). "Rotational and Radial Velocities for a Sample of 761 HIPPARCOS Giants and the Role of Binarity". The Astronomical Journal. 135: 209–231. Bibcode:2008AJ....135..209M. doi:10.1088/0004-6256/135/1/209.